Ink jet printing systems have been developed and widely put into practical use in recording medium printing fields. When an inkjet printer is used to print, ink droplets discharged from a print head are landed on a recording medium, penetrated and fixed to form dots, with a lot of these gathering dots, an image is formed. This process of formation of dots is important to form a clear image. Absorbing materials such as uncoated paper, coated paper and cloth, and non-absorbing materials such as polyester resin and vinyl chloride resin are used as the materials of the recording media.
Paper which is an absorbing material used in recording media is a hydrophilic material and by nature, tends to have an affinity to water because many hydroxyl groups of cellulose exist therein, so that it absorbs water to swell. For this, a hydrophobic material (mainly, a neutral sizing agent) preventing bleeding is added during the course of producing paper for the purpose of providing water resistance to paper for recording media. Also, a white inorganic pigment (for example, clay and calcium carbonate) is added to paper for recording media to provide whiteness and transparency. Because the grain size of the pigment is smaller than the irregularities of fibers, surface roughness formed by the fibers remains on the surface of the paper and therefore, printing accuracy is limited if the paper is used as it is for printing. The surface of the paper is coated with a pigment primarily containing kaolin and calcium carbonate having a grain size of about 0.1 to 0.3 μm to be smoothened. However, a binder such as a synthetic latex used to fix this pigment fills clearances between the pigment grains to hydrophobicize the surface.
Heavy calcium carbonate (one obtained by milling and classifying natural lime stone) is usually used for coated paper such as coat paper and art paper, and uncoated paper such as high-quality paper and middle-quality paper.
When uncoated paper which is an absorbing material is used as a recording medium, if inkjet ink is an aqueous ink, a coloring material is not retained on the surface of the paper because the aqueous ink is easily penetrated into paper fibers, posing a problem that paper bleeding (feathering) is easily caused, so that only insufficient density is obtained. When the bleeding becomes marked, there occurs the penetration of ink to the rear side. When coated paper which is an absorbing material is used, on the other hand, ink is scarcely penetrated and a coloring material is scarcely fixed because a coat layer exists on the surface of the paper, whereby color bleeding which is nonuniform color mixing at a boundary area between different colors readily occurs so that a clear image cannot be obtained. When a non-absorbing material is used as a recording medium, ink is scarcely penetrated and a coloring material is scarcely fixed, whereby color bleeding readily occurs so that a clear image cannot be obtained.
Patent Document 1 discloses a color inkjet recording method for printing on a recording medium by using a reaction solution containing a polyvalent metal salt and an ink composition containing a pigment and a resin emulsion for the purpose of preventing color bleeding while suppressing printing feathering and printing unevenness.
Patent Document 2 discloses an inkjet recording method for printing on a recording medium by using a reaction solution containing a polyvalent metal salt or polyallylamine and an ink composition for the purpose of improving fixability to the recording medium to obtain an image having excellent scratch resistance and water resistance. Patent Document 2 teaches that a colorant, self-crosslinking polymer microparticles having the property of forming a core-shell type film consisting of a core layer having an epoxy group and a shell layer having a carboxyl group, inorganic oxide colloid, a water-soluble organic solvent and water are contained in the ink composition in the method.
Patent Document 3 discloses an inkjet recording method which uses an aqueous ink containing a coloring material and an alginic acid to record on an inkjet recording medium containing a polyvalent metal compound in the surface layer of an ink-receiving layer formed on a substrate, in order to provide an inkjet recording method which suppresses bronzing after image recording and is capable of high-quality printing having excellent scratch resistance.
Patent Document 4 discloses a reaction solution for inkjet recording, comprising at least (i) a calcium salt or a magnesium salt, (ii) an amine salt of sulfuric acid or an organic acid and (iii) a liquid medium, wherein the concentration of the calcium ion or magnesium ion of the calcium salt or magnesium salt in the reaction solution is 0.2 to 0.8 mal/1 and the concentration of the ammonium ion of the amine salt of sulfuric acid or an organic acid is 0.3 to 4.5 mal/l, in order to provide a reaction solution for inkjet recording which improves bleeding when printing is made on plain paper by an inkjet recording method, does not cause the penetration of ink to the rear side and is capable of forming a good image having fixability and color developability at the same time.
To improve the fixability to paper of the pigment, a denatured pigment obtained by binding a functional group having a predetermined calcium index to the surface of a pigment is proposed (see, for example, Patent Documents 5 and 6). Here, the calcium index indicates a measure of ability to coordination-bind dissolved calcium ions, that is, ability of the functional group which captures the calcium ions. The larger the calcium index is, the more strongly and more effectively the functional group coordination-binds the calcium ions.
The above Patent Documents 5 and 6 teach that when inkjet ink containing the denatured pigment is printed on printing paper, the denatured pigment interacts or binds with calcium carbonate or other divalent metal salts existing in or on the surface of the paper, with the result that the pigment is easily fixed to the paper.